This invention relates to a mass flowmeter designed to operate by the Coriolis principle, with one essentially straight, fluid-conducting Coriolis measuring tube, at least one oscillator associated with and exciting the Coriolis measuring tube, at least one detector associated with the Coriolis measuring tube and capturing the Coriolis forces and/or the Coriolis oscillations generated by Coriolis forces, and a compensating cylinder in which the Coriolis measuring tube is mounted and mechanically connected to the compensating cylinder.
The above states that the mass flowmeter in question incorporates, among other components, at least one oscillator xe2x80x9cassociatedxe2x80x9d with the Coriolis measuring tube as well as at least one detector xe2x80x9cassociatedxe2x80x9d with the Coriolis measuring tube. Typically, the oscillator or oscillators, or at least part of the oscillator(s), and the detector or detectors or at least part of the detector(s), are connected to the Coriolis measuring tube. However, since such connection is not imperative, the term used herein is xe2x80x9cassociatedxe2x80x9d rather than xe2x80x9cconnectedxe2x80x9d.
One generally differentiates between two basic types of mass flowmeters operating by the Coriolis principle, one employing a more or less straight Coriolis measuring tube, the other a looped Coriolis measuring tube. As another differentiating feature, there are mass flowmeters with only one Coriolis measuring tube and those with two Coriolis measuring tubes, in the latter case configured to permit either parallel or in-line flow of the fluid.
More recently, mass flowmeters employing only one essentially straight Coriolis measuring tube have increasingly gained in popularity. Compared to mass flowmeters employing either two straight Coriolis measuring tubes or one looped Coriolis measuring tube, Coriolis mass flowmeters with only one straight measuring tube offer significant advantages. The advantage over mass flowmeters with two straight Coriolis measuring tubes lies primarily in the fact that, unlike mass flowmeters with two Coriolis measuring tubes, single-tube designs do not require a flow divider or flow combiner. Compared to single looped or dual looped Coriolis measuring tubes, the principal advantage of the straight tube design lies in the fact that it is easier to manufacture than a looped Coriolis measuring tube, that there is less of a pressure drop in a straight Coriolis measuring tube than in a looped Coriolis measuring tube, and that a straight Coriolis measuring tube is easier to clean than a looped Coriolis measuring tube.
Nevertheless, all these advantages notwithstanding, mass flowmeters with only one straight Coriolis measuring tube present a variety of problems.
First of all, in a straight Coriolis measuring tube, thermal expansion and stress inherently cause variations in the measuring accuracy as a function of the temperature of the moving fluid. In extreme cases, thermal stress can even lead to mechanical damage such as stress cracks in the Coriolis measuring tube.
The above-mentioned problems with mass flowmeters employing straight Coriolis measuring tubes have already been addressed by industry experts (reference is made in particular to the German patent 41 24 295, the German patent disclosure 41 43 361 and the German patent 42 24 379). The problems have been largely solved, on the one hand, by connecting the Coriolis measuring tube to the compensating cylinder in such fashion that any relative movement in the axial direction is inhibited, whereby the axial distance of the connecting point between the Coriolis measuring tube and the compensating cylinder defines the length of oscillation of the Coriolis measuring tube; and, on the other hand, by positioning the Coriolis measuring tube within the compensating cylinder in a tensile-prestressed state (German patent 41 24 295) and/or by producing the Coriolis measuring tube and the compensating cylinder from materials having identical or nearly identical coefficients of thermal expansion (German patent disclosure 41 43 361), and/or by providing a length-variation sensor capable of detecting changes in the oscillation length of the Coriolis measuring tube and of correcting the measurements for oscillation length and stress variations (German patent 42 24 379). In general, it has been possible to produce a Coriolis-type mass flowmeter employing a single straight Coriolis measuring tube with a measuring accuracy of within about 0.1% (ref. prospectus xe2x80x9cCertification of the Corimass G Instrument for Applications Subject to Calibration Regulationsxe2x80x9d, issued by KROHNE Messtechnik GmbH and Co. KG).
However, mass flowmeters operating by the Coriolis principle and employing one straight Coriolis measuring tube also have an inherent drawback (ref. European patent disclosure 0 521 439):
It is necessary for the Coriolis measuring tube or tubes used in mass flowmeters operating by the Coriolis principle to oscillate under the action of at least one oscillator. It is, after all, the oscillation of the Coriolis measuring tube or tubes and the flow of mass through the Coriolis measuring tube or tubes that generate the Coriolis forces or Coriolis oscillations.
In mass flowmeters employing two straight Coriolis measuring tubes or one or two looped Coriolis measuring tube(s), the Coriolis measuring tubes or the active oscillating sections of the looped Coriolis measuring tubes are identical in design and are so positioned and excited that they oscillate in mutually opposite directions. As a desirable result, the overall oscillating structure has no external vibratory effect. The center of inertia remains stationary, compensating for any forces encountered. It follows that no oscillations are introduced into a pipeline system in which this type of mass flowmeter is installed, so that no pipeline vibrations affect the accuracy of the measurements.
Of course, Coriolis-type mass flowmeters employing only one straight Coriolis measuring tube do not offer the benefit of mutually counter-oscillating measuring tubes. The center of mass does not remain stationary and there is no compensation for impinging forces. As a result, a mass flowmeter of this type, when installed in a pipeline, will transfer vibrations into the pipe which, in turn, can affect the measuring accuracy. Industry experts have already addressed the task of minimizing the introduction of extraneous interferences, i.e. vibrations in the surrounding pipeline structure (ref German patent disclosures 44 23 168 and 196 32 500).
To neutralize the aforementioned problems which are peculiar to Coriolis-type mass flowmeters employing only one straight Coriolis measuring tube, the pipeline system in which the mass flowmeter is installed is often provided with additional clamp-down devices. Typically, the pipe through which the fluid flows to the mass flowmeter and the pipe through which the fluid is carried away from the mass flowmeter are clamped down at spatial intervals corresponding to ten to fifteen times the pipe diameter.
Another proposed approach to solving the aforementioned problems which are peculiar to mass flowmeters operating by the Coriolis principle and employing only one straight Coriolis measuring tube has been to install so-called antiresonators at the point where the Coriolis measuring tube is mounted, which antiresonators should have a resonant spectrum of a bandwidth that matches at least one intrinsic, natural oscillation of the Coriolis measuring tube (ref. European patent disclosure 0 521 439). It has been found, however, that in the case of mass flowmeters which are very accurate to begin with, this approach offers no further improvement in terms of measuring accuracy or error reduction.
Another proposed approach, especially for a mass flowmeter employing only one straight Coriolis measuring tube, has been to mount on the compensating cylinder an equalizing unit symmetrical in design and symmetrically positioned relative to the center of the Coriolis measuring tube (German patent disclosure 197 10 806). That equalizing unit must be so designed that the oscillation amplitude of the compensating cylinder is minimized and preferably close to zero.
It is the objective of this invention to further improve the above-referenced existing design of the Coriolis-type mass flowmeter by addressing the problems, detailed above, that are associated with the use of only one straight Coriolis measuring tube.
The mass flowmeter according to this invention which solves these problems, is basically and essentially characterized by a balancing of both the excitation oscillation and the Coriolis oscillation of the Coriolis measuring tube within the compensating cylinder. The term xe2x80x9cbalancingxe2x80x9d in this case means that neither the excitation oscillation nor the Coriolis oscillation affects the compensating cylinder. In other words, neither the excitation oscillation nor the Coriolis oscillation generates any xe2x80x9ccompensating-cylinder oscillationxe2x80x9d so that the compensating cylinder remains xe2x80x9cquiescentxe2x80x9d and unaffected. This invention thus recognizes and addresses the fact that any further improvement in the design of a Coriolis-type mass flowmeter employing only one essentially straight Coriolis measuring tube is attainable only by keeping the center of mass of the entirety of the components within the compensating cylinder, meaning the center of mass of the complete assembly consisting of the Coriolis measuring tube, the oscillator or oscillators and the detector or detectors, at a fixed, stationary point. If the compensating cylinder houses any additional components, they must, of course, be included in the concept of a xe2x80x9cstationary center of massxe2x80x9d.
There are several specific possibilities to implement this concept of a xe2x80x9cstationary center of massxe2x80x9d.
Given that both the excitation oscillation and the Coriolis oscillation of the Coriolis measuring tube tend to shift the center of mass of the assembly consisting of the Coriolis measuring tube, the oscillator or oscillators and the detector or detectors, these components inside the compensating cylinder must be equipped with an equalizing mass, i.e. compensatory balancing elements produced, dimensioned and configured in such fashion as to keep the center of mass of the entire assembly comprising the Coriolis measuring tube, the oscillator(s), the detector(s) and the balancing elements at a fixed stationary point.
In a preferred embodiment version of the mass flowmeter according to this invention, balancing elements are connected to the Coriolis measuring tube preferably in a symmetrical relation to the center axis of the Coriolis measuring tube. In a symmetrical configuration relative to its center axis, the Coriolis measuring tube is provided with retaining devices to which the balancing elements are attached. The retaining devices must be so configured, and the balancing elements so attached, that both the excitation oscillation and the Coriolis oscillation of the Coriolis measuring tube produce compensatory balancing oscillations in the balancing elements.
It is also possible to use, in lieu of retaining devices and identical balancing elements symmetrically arranged relative to the center axis of the Coriolis measuring tube, retaining devices asymmetrically positioned in relation to the center axis of the Coriolis measuring tube, in which case it will be necessary to vary the dimensions of the balancing elements in such fashion that the center of mass of the entire assembly comprising the Coriolis measuring tube, the oscillator(s), the detector(s), the retaining devices and the balancing elements remains stationary.
In addition, the Coriolis measuring tube of the mass flowmeter according to this invention may be provided with a conventional, centrally mounted balancing or equalizing pendulum. This equalizing pendulum may modify the frequency and amplitude of the excitation oscillation of the Coriolis measuring tube.
A preferred embodiment of the mass flowmeter according to this invention is characterized by a symmetrical balancing assembly provided in the compensating cylinder in an essentially symmetrical arrangement relative to the center axis of the Coriolis measuring tube. This balancing assembly may itself be a system capable of oscillating, consisting of a balancing element and an equalizing spring. For a particular design of such a balancing assembly, reference is made to the German patent disclosure 197 10 806 the contents of which is hereby expressly made a part of this present disclosure.
The balancing assembly referred to above can serve to eliminate or at least minimize any residual xe2x80x9ccompensating-cylinder oscillationxe2x80x9d. Any such residual xe2x80x9ccompensating-cylinder oscillationxe2x80x9d may be attributable to a fluid, flowing through the Coriolis measuring tube, the density of which differs from that of the fluids normally passing through the Coriolis measuring tube. As a rule, mass flowmeters including the mass flowmeter according to this invention are designed for a fluid of a specific density. For the mass flowmeter according to the invention, this implies that the concept of a xe2x80x9cstationary center of massxe2x80x9d must also take into account the respective specific density of the fluid in the Coriolis measuring tube, and that the aforementioned balancing assembly must counteract any impact on the xe2x80x9cstationary center of massxe2x80x9d caused by a density variation of the fluid.